(1) Field of the Invention
This invention relates to electrowinning lead employing an arsenic additive in the electrolyte to reduce lead peroxide formation on the anode.
(2) Description of the Prior Art
Electrowinning of lead from acid solutions has been proposed for years. However, the deposition of PbO.sub.2 on the anode at the same time that lead is deposited at the cathode has been an obstacle in electrowinning lead from acid solutions. Since it is difficult to evolve oxygen at the anode at the lower current densities normally employed in electrowinning, stoichiometric amounts of PbO.sub.2 are typically deposited on the anode as lead is deposited on the cathode.
The PbO.sub.2 deposited on the anode must be removed and reprocessed to produce the desired metallic lead product. However, because PbO.sub.2 is insoluble in most acid or alkaline solutions, it must be reduced either in a chemical or pyrometallurgical reaction to PbO or another lead salt which is soluble in the electrolyte before electrolytic reduction to lead can be accomplished. Further, since PbO.sub.2 is generally formed in plates which adhere to the anode, removal and granulation thereof is typically required for efficient reduction in chemical processes. With pyrometallurgical techniques the anode deposit must be heated to elevated temperatures or in the presence of carbon to reduce the PbO.sub.2 to PbO. Since the amount of lead contained in the PbO.sub.2 is approximately equal to the amount deposited at the cathode during electrowinning, close to one half of all lead put into solution in an electrolyte must be reprocessed.
Evolution of oxygen at the anode prevents formation of PbO.sub.2 because the O.sub.2 is evolved instead of reacting with the lead in solution to form PbO.sub.2. However, the current densities required to evolve oxygen are generally much higher than those necessary to produce good cathode deposits. Further, current densities of 200-500 A/sq. ft. while too low to eliminate the formation of PbO.sub.2, often cause decomposition of the insoluble anodes or cause other problems at electrode connections. Use of an unbalanced electrode arrangement with the anode much smaller than the cathode is sometimes resorted to to facilitate oxygen evolution and reduce lead peroxide reduction. None of the above measures, however, satisfactorily overcomes the problem of lead peroxide formation.